The Ethiopian military will begin the "final phase" of an offensive in the rebellious northern Tigray region, Prime Minister Abiy Ahmed said on Thursday, hours after an ultimatum for Tigray forces to surrender expired.
The government gave the Tigray People's Liberation Front (TPLF) 72 hours on Sunday to lay down their arms or face an assault on Mekelle, the regional capital of 500,000 people.
Rights groups raised concerns for civilians caught up in military operations.
Reuters was not immediately able to reach the TPLF for comment. Claims by all sides have been impossible to verify because phone and internet connections to the region are down and access to the area is tightly controlled.
"The 72-hour period granted to the criminal TPLF clique to surrender peacefully is now over and our law enforcement campaign has reached its final stage," Abiy tweeted, adding that civilians would be spared and that thousands of fighters had already surrendered. The TPLF has denied its fighters are surrendering.
"The last peaceful gate which remained open for the TPLF clique to walk through has now been firmly closed," Abiy said.
Abiy, who won the Nobel peace prize last year for ending a long-running stand-off with Eritrea, called on the people of Mekelle to "disarm, stay at home and stay away from military targets".
"Our National Defence Forces have carefully devised a strategy to bring the TPLF criminal clique to justice without harming innocent civilians, heritage sites, places of worship, development institutions and property," he added.
The conflict pits Ethiopia's federal government against the TPLF, which dominated the country until Abiy took power two years ago. Tigrayans make up about 6% of Ethiopia's population.
Abiy accuses Tigrayan leaders of starting hostilities by attacking federal troops. The rebels say his government has marginalised Tigrayans.
African envoys went to Ethiopia to plead for peace on Wednesday, hours before the ultimatum was to expire.
Thousands of people are already believed to have died and there has been widespread destruction from aerial bombardment and ground fighting since the war began on Nov. 4. Nearly 43,000 refugees have fled over the border to Sudan. TPLF rockets have hit the capital of neighbouring Eritrea.
Daniel Bekele, head of the state-appointed Ethiopian Human Rights Commission, said "extreme caution to avoid civilian harm is of even greater importance, now, at this stage of the conflict".
On Wednesday, Human Rights Watch said both sides must avoid putting civilians in danger. The government's warning did not absolve it of the duty to protect civilians when conducting military operations in the city of Mekelle, the watchdog siad.
"We are also concerned by reports that the TPLF has deployed its forces in heavily populated areas. They need to ensure the safety of civilians under their control," it said.